An amorphous oxide film composed of indium oxide and zinc oxide or composed of indium oxide, zinc oxide and gallium oxide have transparency to visible rays, and has a wide variety of electric properties as a conductor, a semiconductor or an insulator. Therefore, such an amorphous film has attracted attention as a semiconductor film used in a transparent conductor film, a thin film transistor or the like.
As the method for forming this oxide film, a physical film-forming method such as sputtering, PLD (pulse laser deposition) and deposition or a chemical film-forming method such as the sol-gel method has been studied. A physical film-forming method such as the sputtering method has been mainly studied since it is a method which can form a film in a large area at relatively low temperatures.
In order to form an oxide thin film by a physical film-forming method such as sputtering, in order to form a homogenous film stably and efficiently (at a high film-forming speed), a target formed of an oxide sintered body is generally used.
As the target (sputtering target) for forming an oxide film (conductive film/semiconductor film, ordinary amorphous film) composed of indium oxide, zinc oxide and gallium oxide various targets have been proposed.
For example, in Patent Document 1, studies have been made on a production method in which a highly insulating Ga2O3 crystal phase is not generated in an In—Ga—Zn—O sintered body.
Patent Document 2 discloses a target which is composed mainly of a homologous structure represented by InGaZnO4(InGaO3(ZnO)) (i.e. an XRD peak derived from other metal oxides is not observed) and has an atomic ratio of In, Ga and Zn is 1:1:1.
Patent Document 3 discloses a polycrystalline oxide target containing a homologous structure represented by InGaZnO4(InGaO3(ZnO)).
In particular, a target formed of a single crystal form (a target in which an XRD peak derived from other metal oxides is not observed) has been desired. It is because a target formed of a single crystal form is expected to have advantages that quality control is easy, abnormal discharge occurs less frequently, deviation in composition ratio between the target and the thin film is small, a strong film can be easily formed, or the like.
However, in order to produce a target formed of a single crystal form (a target in which an XRD peak derived from other metal oxides is not observed in an X-ray diffraction), it is believed that an atomic ratio of In and Ga of 1:1 is required. Therefore, studies were made on a target in which the atomic ratio of In and Ga is 1:1 (for example, the atomic ratio of In, Ga and Zn is 1:1:1, 2:2:1 or the like).
In studies using these targets, with a progress in studies for mass-production, problems that the film thickness varies according to a slight distribution of plasma density at the time of sputtering, the film thickness varies according to a slight variation in oxygen partial pressure between batches or the like have come up to the surface. The reason therefor is assumed to be as follows. Since the bonding power with oxygen is too strong, when an attempt is made to form a film having a carrier density suited to semiconductor applications, it was necessary to form a film at a film-forming speed which is sensitive to a variation in oxygen partial pressure or the like. The reason therefor is assumed to be well-balanced atomic ratio of In and Ga and an excessive amount of Ga in these targets.
On the other hand, even in the case of a target in which the atomic ratio of In, Ga and Zn is 1:1:1, it was difficult to produce a target formed of a single crystal form. As shown in Patent Document 1, in order to suppress the generation of a crystal form such as Ga2O3, many restrictions are imposed on the manufacturing conditions. Further, there are many cases that, in a crystal which is judged to be a single crystal, other crystal peaks are observed when observation is made in detail. For example, an invented product A of Patent Document 3 has a relatively low density. However, FIG. 5 shows that this product A has a peak which is not attributed to InGaZnO4 in a range of 2θ of 62.0 to 62.6 degrees in X-ray diffraction. Further, as shown in Comparative Example 2 of Patent Document 1 and in Comparative Example 1 of Patent Document 2, a problem may tend to arise that a target having a single crystal form, even if it is produced, has a low density, a high resistance or the like.
On the other hand, Patent Document 4 discloses an In—Ga—Zn—O target with a composition in which the Ga content is small. However, studies on the production method or the properties of the target are not insufficient, and the resistance of the target disclosed in this document is high. Therefore, studies on production of a target formed of a single crystal form with a composition ratio having a low Ga content have not been made.
Studies on the synthesis of various crystal forms using oxide powder were made (Non-Patent Document 1). However, no studies were made on a sputtering target which is formed by sintering a shaped body. Further, no attempts were made to prepare a thin film transistor.